1. Field of the Invention
The present invention relates to an optical fiber having an improve hydrogen-resistance property.
2. Description of Related Art
The hydrogen-resistance property is one of the properties of an optical fiber made of silica glass, which are important for transmission characteristics of the optical fiber.
FIG. 11 is a graph indicating the relationship between wavelength and loss before and after hydrogen exposure of a typical optical fiber under conditions in which it is exposed to 100% hydrogen at 30xc2x0 C. for 21 hours. In the following, the hydrogen exposures are carried out under the same conditions.
In FIG. 11, curve A shows the amount of the loss before exposure, and curve B shows the amount of the loss after exposure. In order to clarify the effects of the hydrogen exposure, the loss increase before and after hydrogen exposure is indicated by curve C in FIG. 12.
By the exposure to hydrogen, a high peak of loss increase is generated near 1530 nm, and a low peak of the loss increase is generated near 1580 nm. The peak near 1580 nm is caused by only the effect of hydrogen, and the peak near 1530 nm is caused by a combined effect of hydrogen and peroxyl radicals existing in an optical fiber as described below.
Curve D shows the loss increase calculated by subtracting the loss increase caused by only the effect of hydrogen from the loss increase shown by the curve C, in which the low peak near 1580 nm is almost extinguished, and the high peak near 1530 nm relatively remains.
Since the wavelength used for multiple purposes in a general optical communication system is within the so-called C band region (1530 to 1560 nm), sequential changes of the loss caused near 1530 nm by hydrogen significantly affect transmission properties of an optical fiber.
In recent years, Wavelength Division Multiplexing (WDM) systems using a wavelength region centering at 1550 nm are often used.
An amplifier system, which compensates loss wavelength properties of an optical fiber in a broad region, for example, of 1530 to 1560 nm, is previously used in this system.
However, when the loss wavelength properties are sequentially changed by hydrogen entering the optical fiber, the amplifier system cannot compensate for the loss wavelength properties of the optical fiber, thus resulting in it significantly affecting the entire system.
The loss peak near 1530 nm is generated by the following effects.
When a peroxyl linkage represented by the following chemical formula:
xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Si xe2x95x90xe2x80x83xe2x80x83Chemical Formula 1 
exists in an optical fiber preform produced under an excessive oxygen atmosphere, the peroxyl linkage is decomposed to yield peroxyl radicals represented by the following chemical formula:
xe2x89xa1Sixe2x80x94Oxe2x80x94.xe2x80x83xe2x80x83Chemical Formula 2 
in an optical fiber produced by melting and spinning from the optical fiber preform under certain conditions. When hydrogen enters this optical fiber, it reacts with the peroxyl radicals to yield Sixe2x80x94Oxe2x80x94Oxe2x80x94H species which cause the loss near 1530 nm. The Sixe2x80x94Oxe2x80x94Oxe2x80x94H species finally loses an oxygen atom to form Sixe2x80x94OH species which absorbs at 1380 nm. Once the Sixe2x80x94OH species is formed, even when it is exposed to hydrogen, absorption at 1530 nm is not caused.
In order to suppress the loss increase, a method in which conditions for melting and spinning are optimized to reduce generation of peroxyl radicals in an optical fiber is proposed. Moreover, a method in which an optical fiber is pretreated under a hydrogen atmosphere is proposed.
However, these methods have various problems in that manufacturing equipment is limited and complicated steps are required to produce the optical fiber.
With regard to the structure of an optical fiber, various studies have been carried out. For example, Japanese Unexamined Patent Application, First Publication No. Hei 9-15464 discloses an optical fiber including a core produced by sequential lamination, a vapor deposited clad layer, and a tube leading-out clad layer, in which the tube leading-out clad layer has a hydrogen getter site including materials selected to trap hydrogen in order to substantially prevent the diffusion of hydrogen into the vapor deposited clad layer at the time of producing the optical fiber.
However, the optical fiber has various problems in that the production method is limited, and it cannot be used to provide a multipurpose refractive index profile.
Japanese Unexamined Patent Application, First Publication No. Hei 9-171120 discloses an optical fiber including a core produced by sequential lamination, an inner clad, and an outer clad, in which germanium is added into the inner clad to which propagating optical power spreads from the core, so as to prevent formation of the peroxyl linkage, which is represented by the Chemical Formula 1 described above and is caused by excess oxygen, and the loss caused by reaction between the hydrogen and the peroxyl linkage.
However, Japanese Unexamined Patent Application, First Publication No. Hei 9-171120 merely vaguely discloses that a considerable amount of light propagates to the inner clad region doped with germanium. Moreover, only a specific refractive index profile in which the inner clad region and the outer clad region are made from essentially the same material is studied. Therefore, the optical fiber disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-171120 cannot simply be applied to various refractive index profiles as now proposed.
Specifically, progressive change of the loss near 1530 nm tends to affect in a WDM system. In contrast, although an optical fiber having a relatively complicated refractive index profile is proposed to be suitable for a WDM system, it tends to be difficult to apply this complicated refractive index profile to a conventional method and to provide a stable system.
The present invention is achieved in view of the circumstances described above. An object of the present invention is to provide an optical fiber having an improved hydrogen-resistance property.
An object of the present invention is to provide an optical fiber in which the loss peak specifically near 1530 nm caused by linkage between peroxyl radicals and hydrogen is suppressed.
Moreover, an object of the present invention is to provide an optical fiber which can be applied to various refractive index profiles, and has a hydrogen-resistance property which suppresses the loss peak specifically near 1530 nm.
In order to solve the problems described above, the present invention provides an optical fiber comprising a high concentration germanium layer and a low concentration germanium layer, wherein the high concentration germanium layer is disposed at a central position of the optical fiber and contains germanium oxide in a concentration of 0.1% by weight or more, relative to the total weight of the high concentration germanium layer, the low concentration germanium layer is disposed around the high concentration germanium layer and contains germanium oxide in a concentration of less than 0.1% by weight, relative to the total weight of the low concentration germanium layer, and the ratio of optical power leaking from the high concentration germanium layer to the low concentration germanium layer in an employed wavelength band is 0.4% or less, relative to the total optical power propagating through the optical fiber.
Moreover, the present invention provides an optical fiber comprising a high concentration germanium layer and a low concentration germanium layer, wherein the high concentration germanium layer is disposed at a central position of the optical fiber and contains germanium oxide in a concentration of 0.1% by weight or more, relative to the total weight of the high concentration germanium layer, the low concentration germanium layer is disposed around the high concentration germanium layer and contains germanium oxide in a concentration of less than 0.1% by weight, relative to the total weight of the low concentration germanium layer, an external diameter of the high concentration germanium layer is at least 2.6 times that of a mode field diameter in an employed wavelength band.
In the optical fiber, the low concentration germanium layer may include a clad, the high concentration germanium may include a core and an intermediate layer disposed between the core and the clad, and the maximum refractive index of the core may be 0.25% or more higher than that of the intermediate layer.
In the optical fiber, a dopant other than germanium oxide may be added together with the germanium oxide in the high concentration germanium layer.